Information storage and retrieval

ABSTRACT

An apparatus for recording information in the form of a two dimensional array of miniature images on a recording material such as electrophotographic, photographic or photochromic material. Means are included for forming those images (by such of the processes of sensitizing, charging, exposing, developing or fixing as are appropriate to the particular material used) from originals or copies of legible size. A facility is provided to form one or more miniature images occupying only part of a storage member on one occasion, and to record a further such image or images on later occasions. Guide means and propulsion means in two dimensions are included for adjusting the relative positions of the storage member and the image forming or image viewing device so that any image space may be presented to the image forming device and any image may be presented to an image viewing device. Two dimensional measuring means are also included which provide information to a control system of the propulsion means so that it can present any chosen image space or image, defined by two numbers which are coordinates, to the image forming device or the image viewing device.

This invention relates to information storage and retrieval involvingoptical storage.

In an information storage system, it is a common requirement to input aquantity of information less than the total capacity of the system, toindex that information for easy retrieval and to refer to thatinformation (i.e. to use it) using the index. At a later date moreinformation may be input and the process repeated until the storagecapacity of the system is full or nearly full. It may then be possibleto extend the storage capacity. This type of use is exemplified by aconventional filing cabinet in which documents may be placed and indexedin such a manner that they can be found easily, and in which moredocuments can be added as they come to hand.

Because of the bulk of large quantities of paper, it is now common tostore information in various forms more compact than the originalcharacters or graphical representations on paper. Common forms of highdensity information are small-scale reproductions of the original,encoded forms of the original such as video (television-type) encodingand digital representations of the characters constituting the original.

Two different optical storage systems have hitherto found commercialacceptance, one employing microfilm and the other employing opticallaser storage methods.

In microfilm systems, high capacity optical data storage is effectedphotographically on light-sensitive microfilms, such as microfiche.Capacities of up to ten million bits/mm² have been achieved, the maximumbeing limited by the quality of the photographic emulsions available.The production of microfilms for a very high capacity system istherefore very expensive. Microfiche is one type of microfilm systemwhich involves reproducing a large number of pages of information on asingle sheet which can be searched manually in two dimensions. Such amicrofiche system is disclosed in U.S. Pat. No. 4,179,743. In additionto storing pages, the system stores adjacent each page a marker relatedto the content of the page. That system is therefore applicable only tostoring information which is intrinsically ordered or self-indexed (inthe manner of a dictionary).

Optical laser storage systems using discs have been made whereininformation is stored in the form of a coded series of pits inconcentric rings, or in spirals, on the surface of a transparent disc.The information is read using a source of coherent laser illuminationfocussed on each of the pits in turn as the disc is rotated. The spotsize of the illumination must be less than or comparable to the area ofa single pit. The light diffracted by the pits, either on reflection orin transmission, is monitored and is used to reconstruct electronicallyan image of the stored information. In this case the information isnecessarily encoded and so is not directly readable, requiring the useof electronic decoders to convert the stored information into signalssuitable for a video display unit.

A disc system has been proposed with which it is possible to imageoptical information without the use of laser light and somewhat in themanner of microfilm or microfiche. Such a system is disclosed in BritishPatent Specification No. 1091981 and comprises the use of a disc havinga spiral groove and, between the turns of the groove, a flat spiral landhaving a coating of photographic emulsion constituting a video track. Areading head is employed which has a pick-up which physically engagesthe groove for the purpose of accurately locating the reading headrelative to the video track. Video data is recorded on the track so thata flying spot scanner can scan picture elements in a linear fashion.

An object of the present invention is to provide an optical informationrecording arrangement which permits information to be recorded inrelation to an absolute position measuring system, enabling items ofinformation to be recovered readily by a control system utilising thatmeasuring system.

An object of preferred embodiments of the present invention is toprovide an optical information recording arrangement which permitsinformation to be recorded in an interrupted fashion, i.e. so that theinformation holding member can be used between successive recordingoperations.

According to one aspect of the present invention, there is provided arecording apparatus for recording optically readable images on a storagemember comprising recording material by means of which such images canbe recorded on the member, the apparatus comprising processing means forrecording said images on the storage member the processing means havingimaging means for imaging data to be recorded onto the storage member,and the apparatus also comprising drive means for producing relativemotion between the imaging means and the storage member so that an imagecan be directed at a desired location of the storage member,characterised in that there are control means for controlling the drivemeans to enable an image to be recorded at any desired one of aplurality of areas of a two-dimensional array of areas in a first zoneof the storage member, the control means comprising for readingpositional information conveyed by the storage member in a second zoneto ascertain a datum position for a first co-ordinate and a secondco-ordinate of a two-dimensional co-ordinate system in which each ofsaid areas has its own two-dimensional address.

According to a second aspect of the invention there is provided astorage member comprising recording material by means of which opticallyreadable images can be recorded and stored by the member, characterisedin that the member comprises a first zone at which said recordingmaterial is provided and, distinct from the first zone, means whichprovide a machine readable datum for a first co-ordinate and for asecond co-ordinate of a two co-ordinate system providing addressingmeans for each of a plurality of areas of the first zone.

In a preferred embodiment, the storage member has a second zone or zonesdistinct from said first zone and carrying pre-recorded absolute addressdata in one of the co-ordinate directions. Position in the secondco-ordinate direction may be provided by a pre-recorded position elementor elements pre-recorded in a zone also distinct from said first zone.The zone which comprises the information recording areas may bephotographically, electrophotographically or photochromically sensitiveand appropriate image developing and/or fixing means will be providedaccording to the type of zone involved.

The apparatus may also function as a reading apparatus for readingrecorded images in the information recording areas, for example inaccordance with European Patent Application No. 81300321.7.

Possible embodiments of the invention will now be described.

These embodiments employ a storage member which comprises a plurality ofinformation recording areas at each of which an optically readableanalogue image can be retained by at least the step of directing anoptical analogue image onto that area. For this purpose, material atsaid areas may be electrophotographic, photographic or photochromic.

Each area can have a distinct image retained at it so that, preferably,the information is held in the form of individual pages in respectiveareas. The areas can be processed at different times so that atwo-dimensional area of miniature images can be built up by the userover a length of time.

Information is preferably recorded on the storage member in the form ofreduced images by electrophotography. The individual pages, images orareas can, for example, be arranged in concentric rings or in a circularspiral. Alternatively, the individual pages of information can bearranged in zones having other geometrical forms.

Information is preferably recorded on the optical storage member in theform of reduced images formed by electrophotography. The individualpages, images or areas can, for example, be arranged in concentric ringsor in a circular spiral. Alternatively, the individual pages ofinformation can be arranged in zones having other geometrical forms.

Any one of several electrophotographic systems may be used to effect therecording of information. The following are given by way of example.

1. TEP (transparent electrophotographic material). For this method, thesubstrate consists of a thin sheet of a moderately rigid material (forexample polyethylene terephthalate which may be about 60 microns thick).The exact thickness and choice of material for the substrate are notcritical. The substrate carries a very thin layer of an electricallyconducting material. In some embodiments, this conducting layer issufficiently thin to be optically transparent. Another layer, overlyingthe conducting layer, is a thin layer of a photoconductive material.This layer, which may be about 10 microns in thickness, can be anorganic photoconductive material, for example polyvinyl chlorideincorporating a small proportion of a dye coupling compound.Commercially available materials of this sort are Kodak (RegisteredTrade Mark) materials S0101 and S0102. Alternatively, thephotoconductive layer may be an inorganic photoconductive material suchas cadmium sulphide. In order to form an image on TEP material, anelectrical charge is first applied to the surface of the material. Forthe Kodak materials mentioned above, a positive charge is applied bymeans of a Corotron so as to raise the surface potential of the materialto a value in the range 500 to 800 volts with respect to the conductivelayer (which may be earthed). A convenient method of uniformly charginga sector of the surface is to rotate the surface at uniform speed,adjacent to a linear Corotron which is arranged radially with respect tothe optical storage disc. The Corotron then delivers a distributedcharging current to the surface, the magnitude of which current isproportional to the radius where the Corotron is located. After thesurface has been charged, an optical image is projected by a suitableoptical system onto that part of the surface on which a page or pagesare to be recorded. When the surface is illuminated by parts of theprojected image, at least part of the surface charge leaks away. Wherethe surface is not illuminated, most of the surface charge remains.Toner is then applied to the area where the image has been projected.The toner can conveniently consist of very fine black powder insuspension in an electrically insulating fluid, for example a paraffinor mixture of paraffins. Depending upon the nature of the black powderselected, it may either adhere to the regions where most charge remainsor to the regions which are most nearly discharged, thus giving either apositive or a negative image. Excess toner over and above that adheringto the electronic image is then removed and the image area is heated tofuse the tones into a mass which is strongly adherent to thephotoconductive layer.

2. Migration imaging. The material for forming a migration imagecomprises a substrate, which may be polyethylene terephthalate and whichcan conveniently be 50 microns thick. Superimposed on the substrate is avery thin layer of an electrically conducting material. This layer canbe sufficiently dense for the material to be optically transparent.Located above the tin conducting layer there is a thicker layer of aninsulating thermoplastic material. Particles of an opaquephotoconductive material are located within this layer, and can be, forexample, located as a mono-layer just beneath the upper surface of thethermoplastic material. With this arrangement, the mono-layer iscontinuous and substantially optically opaque. Typically it may transmitno more than 1% of incident light. In order to form an image in suchmaterial, an electrical charge is deposited on the surface of thematerial in a manner similar to that described above for the TEPmaterial. An optical image is then projected onto that part of thesurface where a page or pages of information are to be recorded. In thebright parts of the image, the photoconductive particles becomeconducting and take up charge from the thermoplastic material. Afterexposure, the thermoplastic layer is heated until is softenssufficiently to allow the particles a limited mobility. In thiscondition, the charged particles move by electrostatic attractiontowards the earthed conducting layer against the viscous resistance ofthe thermoplastic material. After this migration, the thermoplasticlayer is allowed to cool, thus freezing the particles in position. Thisprocess causes disruption of the mono-layer into a three-dimensionaldispersion of particles the form of which corresponds to the bright anddark areas of the illuminated image. The dispersion of particles permitsthe passage of a proportion of incident light, the maximum beingtransmitted when the particles are distributed throughout the entirethickness of the layer. With this migration imaging technique, theheating step must not be too long, since this would permit reforming ofan opaque mono-layer on the conducting layer, and would thus reduce thecontrast of the image.

The above image forming methods are not intended to be exhaustive, andthe present invention can operate with other image-forming techniques.

Returning now to general aspects of preferred embodiments, it is pointedout that, prior to use, said information storage areas of the storagemember need not be distinct preselected areas but could beindistinguishable portions of a recording zone at substantially anypoint at which an image may be recorded. The location of individualpages or areas is then determined, during recording or later retrieval,by measuring means of the recording or retrieval apparatus. For thispurpose, the storage member has recorded on it at least one machinedetectable datum defining a datum for one co-ordinate direction and adatum for a second co-ordinate direction of a two-co-ordinate measuringsystem by which the location of each area or page can be defined bymeasuring means. A datum may be, for example a detectable trackextending in the intended direction of the first co-ordinate, thusproviding a datum or "zero" position for the second co-ordinatedirection. Such a track thus defines the direction of one co-ordinateand the datum for the other. A datum may be provided at, or separatefrom, such a track to define the datum or "zero" for the firstco-ordinate direction. All control data, such as a co-ordinate datum,are in zones of the storage member distinct from the zone or zonescontaining the information storage areas. In a preferred embodiment,there may be a control data zone containing absolute address data in oneof the co-ordinate directions. Moreover position data may be provided todefine a dimension of the storage member for use in monitoring thatdimension of the member, e.g. to compensate for thermal dimensionalchanges of the storage member.

However the datum is provided, two-dimensional measuring means areprovided in the system to provide information by which any chosen area,defined by a two-co-ordinate address, can be presented to image formingmeans for recording, or to image viewing means for retrieval.

Coming now to the preferred form of apparatus for recording informationon the storage member, it comprises a support carrying image formingmeans (and possibly image viewing means) and drive means for producingrelative motion in the two co-ordinate directions between the storagemember and the image forming means (and any image viewing meansprovided).

In one embodiment, first drive means are provided to rotate the storagemember relative to the support and second drive means are provided todrive the storage means relative to the image forming means in adirection passing through the axis of rotation of the storage member. Inan alternative embodiment, the second drive means displaces the imageforming means radially of the storage member.

The above-mentioned measuring means may measure the displacementproduced by the two drive means independently of the storage member toobtain actual value positional information relative to the datum of eachco-ordinate direction. However, in a preferred embodiment only thedisplacement produced by one drive means is measured in this way, e.g.the second drive means. In that case, the measuring means for the otherdimension employs means for reading positional information provided onthe storage member itself in respect of that other dimension, e.g. theangular co-ordinate for the situation in which the storage member isrotated in use.

Nevertheless, there may be positional information for each dimensionprovided on the storage member, so that the latter fully defines its ownco-ordinate system and addresses, this providing more accuracy thanmeasuring displacements separately from the storage member. However,more information storage areas are available if, in at least onedimension, the positional information, other than the datum, is notprovided by the storage member itself.

The apparatus of preferred embodiments also has means for writingdigital data on the storage member, preferably optically and preferablyby the same storage methods as for the analogue information. These meansmay be additional to the image forming means to take into account thefact that individual digital images may occupy an area which issignificantly smaller than that occupied by an analogue page.Nevertheless these additional means may include elements of the imageforming means. Means may also be provided to read digital data carriedby the storage member and to convert that data into a form suitable forinput to digital computing means.

Digital data is preferably grouped on the storage member in a zone orzones distinct from the zone or zones containing the information storageareas. Preferably such a zone or zones containing digital data isprovided by a track or tracks extending in one co-ordinate direction ofthe storage member.

In the case in which digital data is stored, means may be provided toenable a user to supply a name, or code representing names, from whichthe apparatus ascertains the corresponding address from the digitalinformation contained in the storage member and then causes theappropriate area to be presented to image viewing means.

Preferred embodiments also include means to determine to co-ordinates ofan area or areas defined by a logical combination of names, or codes, orclasses of information.

With regard to the accuracy of locating specific areas of a storagemember for recording and retrieving information, analogue or digital, itwill be apparent there is a co-ordinate system implicit in the storagemember and a co-ordinate system implicit in the drive means producingthe necessary relative motions (and the measuring means if any, which,independently of the storage member, measure the displacements producedby the drive means). In theory, these co-ordinate systems will beidentical, but three types of discrepany may exist in practice:

(i) Position error, e.g. eccentricity when the co-ordinate system ispolar, and skew when the co-ordinate system is Cartesian;

(ii) Isotropic scale error, e.g. due to thermal expansion differencesbetween the storage member and separate measuring means if provided;

(iii) Anisotropic scale error, e.g. due to anisotropic expansion, forexample with a multilayered record member. This produces a secondharmonic error in angle in a polar co-ordinate system and anorthogonality error in a Cartesian system.

All these errors are readily determined by measuring the apparentco-ordinates of a number of datum points of the storage member. Once theerrors are determined a co-ordinate transformation can be applied to theexpected co-ordinates of any position specified. In a preferredembodiment, at least discrepancy (i) is taken into account and in thecase in which, in at least one dimension, displacements are measuredseparately from the storage member, discrepancy (ii) is also compensatedfor. Depending upon the nature of the storage member and the resolutionrequired, discrepancy (iii) may additionally be taken into account.

Fluid pressure (aerostatic or hydrostatic) bearing means are included inpreferred embodiments accurately to locate the part of the storagemember in use relative to the image forming and/or viewing means.Adjustment of the bearing means is infinite over a certain range in thefocussing direction relative to the image forming and/or viewing means.Such bearing means provide means for maintaining the storage member outof contact with its processing means whilst it is in use andadditionally, when electrical charges are employed during image forming,such bearing means maintains the member out of contact with the elementsto which it might otherwise electrically discharge.

According to a third aspect of the invention, there is provided a methodof storing information on a disc, which comprises (a) sensitizing apredetermined zone of the disc so that it becomes sensitive toelectromagnetic radiation in, or close to, the optical range ofwavelengths; (b) focussing onto the said predetermined zone, or a partthereof, a reduced image of the information to be recorded; (c)developing and/or fixing the predetermined zone or said part thereof soas to hold the reduced image in reproducible form within or on thematerial constituting the predetermined area of said disc; and (d)providing positional address data on the disc in one or more areasseparate from said predetermined zone to enable the information recordedat said zone to be retrieved. At least some elements of the address datamay be pre-recorded on the disc before steps (a), (b) and (c) above areperformed. In one embodiment, the disc has a first area or areascarrying pre-recorded absolute addresses in one co-ordinate directionand the predetermined zone or (zones) where information is to berecorded are located apart from said first area or areas. Address datain a second co-ordinate direction may be provided by a pre-recordedposition element or elements pre-recorded in an area also distinct fromsaid predetermined zone or zones. In another embodiment, address data isrecorded on the disc in predetermined areas thereof after steps (a), (b)and (c) above have been carried out; with such an arrangement, it ispossible to record a "page" of information in part of a predeterminedzone of the disc, then to record address data relating to that page, andthereafter to record a further page of information in a second part ofthe said predetermined zone of the disc. The sequence of recording stepsinvolved, and the location of successive pages of information stored inthe predetermined zone or zones of the disc will be determined accordingto the arrangements adopted for holding address data.

In carrying out step (a) of the above method, the electromagneticradiation involved will generally be in the visible range ofwavelengths. The method of the invention may, however, be performedusing near ultra-violet or near infra-red radiation instead of, or aswell as, visible radiation. The material located at said predeterminedzone and which, after recording, will hold an image of the informationbeing stored, can be a transparent electrophotographic material or amigration imaging material as described hereinabove. Other forms ofimage-holding materials may be used.

The developing and/or fixing step in the above method will generallyinvolve the application of a toner, which may be for example a liquidtoner comprising an electrically insulating liquid having dispersedtherein fine black powder particles. After application of the toner,excess toner material is removed and, at this stage, the predeterminedzone of the disc carries a positive or negative reduced image of theoriginal information focussed onto it. This image is then madepermanent, for example by heating the predetermined zone of the disc soas to fuse the toner particles thereon.

Apparatus in accordance with the present invention will generallyinclude a number of sub-assemblies such as, for example, a system forholding the information storage member precisely in place during therecording of an image, and a servo system for controlling relativemovement between the optical storage member and the means for directingan image onto that member. These sub-assemblies may be constructed inaccordance with the disclosure given in our European Patent ApplicationNo. 81300321.7.

For a better understanding of the invention and to show how the same maybe carried into effect, reference will now be made, by way of example,to the accompanying drawings, in which:

FIG. 1 is a schematic view in plan of a data storage member;

FIG. 2 is a view of coded data on the member of FIG. 1;

FIG. 3 is a plan view of a recording and retrieval apparatus;

FIG. 4 is a schematic side elevation of part of an optical system foruse in the apparatus of FIG. 3;

FIG. 5 is a diagram of a possible optical path for use in the recordingand display of information using the device of FIG. 3;

FIG. 6 is a block diagram of a control system for a recording andretrieval apparatus;

FIGS. 7 and 8 are diagrammatic plan and elevational views, respectively,of a toner applicator forming part of the apparatus of FIG. 3;

FIGS. 9 and 10 show, in plan view, alternative toner applicators;

FIGS. 11 and 12 are diagrammatic plan and cross-sectional views of twopreferred toner applicators;

FIG. 13 is a side elevation of a toner applicator and applicator movingequipment for use in the device of FIG. 3.

FIGS. 1 to 13 show an embodiment of an optical data storage systemcomprising optical data storage members in disc-form and an optical datarecorder and reader.

FIG. 1 illustrates an example of an optical data storage member 1 inschematic plan.

The member 1 is a circular disc of TEP material for example of about 30cm diameter.

A hole 2 is provided in the central area 3 of the disc for locating thedisc in the data recorder and reader. The illustrated example of disc ispreformatted with optically readable tracks 4,5 and 6 of control data,tracks 5 and 6 being at the circumference, track or tracks 4 encirclingthe central area 3 and the track or tracks 4 being separated from tracks5 and 6 by an area 7 which, prior to first use, contains no data orimage and which is intended to have recorded in it optically displayableanalogue information in substantially rectangular areas 8, each sucharea being referred to herein as a `page` of information These pages 8are arranged in concentric circles. In principle, any arrangement of thepages is possible such as spiral, square grid or even random, but anarrangement with concentric circles enables a simpler control system tobe provided to locate individual pages.

The central area 3 is of about 5 cm radius and is left substantiallyfree of data and circumferential tracks 5 and 6 carry analogue and/ordigital coded page locating and control data. The control data isintended to be used by positional servos to provide page access. Theform of this control data will be described later with reference to FIG.2. The track or tracks 4 contain digital data or can have digital datarecorded on them and will be described later.

FIG. 2 is a diagram of the control data tracks in developed form. Each`bit` of data is hatched in this figure and is substantially opaque, theplain areas which are adjacent being light transmissive i.e. `white`.The reverse is also possible, when the hatched areas are `white`.

At the periphery of the disc is an incremental, angular position, track6 consisting of a regular series of `bits` at a spacing of 130.2 μm andwith a width of 2 to 3 μm. A fixed moire fringe transducer willcontinually monitor this track to provide a measure of the angulardisplacement of disc 1.

Next to track 6 in FIG. 2 is an absolute angular position track 5 (analternative position 5' is shown just above area 3). This trackcomprises a pair of guard rails 9 and 10 between which are sets 11 of`bits`, each set being a radially extending 20 bit character comprising16 data bits and 4 control bits, e.g. providing a check digit. Each setcomprises hatched areas representing digital `1` and unhatched areas(shown dotted) representing digital `0`. There are 6000 sets, with oneset aligned with each successive pair of bits in track 6. This track 5therefore defines the absolute angular disc position every 260.4 μm.Circular rails or tracks 9 and 16 serve the purpose of defining thelimits of the data and also enable disc eccentricity to be measured.During one complete revolution of the disc the rails can be scanned andat some or each set of bits 17 an eccentricity value can be measured andstored to provide eccentricity compensation during servo positioncontrol. Track 10 is about 5.6 μm wide but track 9 is much wider, e.g. 1mm, in order that it should unambiguously be detected as its function isto provide a measure of eccentricity, a datum for a radial co-ordinateand, in conjunction with a wide band 12, to provide a preset radialdistance measure.

One or more data tracks 4 are also provided and have the same digitalstructure as track 5.

It will be apparent that all these bits of FIG. 2 could be provided, forexample, as magnetic or photographically produced markers to be readmagnetically or optically respectively.

In a preferred embodiment, tracks 5 and 6, and all the rails or tracks9, 10 and 12 are preformatted by contact printing for optical reading. Aportion of digital data in track or tracks 4 may also be preformatted bycontact printing or by digital writing techniques as described hereafterfor track or tracks 4. Track or tracks 4 will be written with furtherdigital data by a user.

In general terms, track 6 provides what may be termed as a co-ordinatescale which enables increments of movement to be measured, in this caseannular increments. Another scale is provided in the recorder itself (tobe described hereinafter) to provide a measure of radial increments ofmovement; these scales are described and shown as equally spacedgraduations to be read optically, but magnetic or other suitable meansknown in the art of metrology are possible.

In addition to these scales there is provided an absolute datum for theincremental measurements. Thus, one of tracks or rails 9 and 12 providesan absolute datum for the radial measurements. The rails 9 and 12together provide a fixed radial measurement by which radial measuringmeans may be calibrated. Moreover, track 5 provides an absolute datumfor the annular measurement; in fact it provides 6000 alternativeabsolute data.

One form of a recorder, with a retrieval facility, will now be describedin more detail with reference to the use of TEP as the recordingmaterial, i.e. a material comprising a transparent substrate carrying atransparent conductive layer on top of which is a transparentphotoconductor. At the centre of the disc a small portion of thephotoconductor is removed from the conductive layer to expose it and toenable an electrical connection to be made to it to a virtual earth.

Reference will now be made to FIG. 3 which is a plan view of therecorder with its optical system not shown for clarity.

The recorder comprises a base generally denoted 14 and which can beregarded as the general area of the paper in FIG. 3. Uprights 15, 16, 17and 18 extend upwardly from the base and support two linear guide rails19 and 20 which are in spaced-apart parallel relationship. On theseguide rails is supported a carriage, generally denoted 21, whichcomprises a plate or tray 22 having an aperture 23 of a diameterslightly larger than that of a disc to be processed. The carriageincludes a transverse wall 24 which supports the nut 25 of a lead screwand nut arrangement comprising a lead screw 26 rotatably mounted in theuprights 17 and 18 and driven by a controllable motor 27. The transversewall 24 also supports a bearing arrangement 28 comprising a verticalspindle, rotatable in bearings, made of an electrical insulator. Thedisc is in use placed on top of the spindle, active side down, and aclamping cap 29 is applied to hold the centre of the record down on tothe top of the spindle and to locate it by means of a spigot extendingfrom the cap through the hole in the record and into the verticalspindle. The transparent electrode of the disc material makes electricalcontact with the cap and thence with a conductor which is connected by aslip ring to the virtual earth of a virtual earth amplifier.

The lower end of the spindle carries a metal disc 30 the rim of which isfitted with a rubber tire 31. The disc 30 acts both as a fly wheel andas one member of a friction type reduction gear. The other member of thereduction gear is a plain diameter on the spindle 32 of a drive motor 33which provides the angular drive for the disc. The motor 33 is pivotallymounted to the transverse wall 24 and is biassed by a spring 34 againstthe tire 31.

A moire grating reading head 35 is mounted on a swinging arm 36 pivotedat 37 to the tray 22, in order that it should be moveable between twopositions. In its illustrated operating position it co-operates with theradial grating provided by the track 6 of the disc to measure theangular increments of motion of the disc. In its other position, the armand reading head are clear of the disc to facilitate exchanging one discfor another. A linear grating 38 is mounted to the tray 22 andco-operates with a reading head 39 which is attached to the staticframework of the recorder to provide the second scale.

Fixed to the static framework of the machine is an optical system whichis not shown in FIG. 3. However, a dotted circle 40 is shown in FIG. 3and this represents the optical path of the optical system through asupported disc when the tray 22 is in its rightmost postion as shown inFIG. 3.

A second dotted circle 41 illustrates the operating work stationposition for processing images to retain them on a disc. For thispupose, the recorder is provided with a toner 42 and a fuser 43. Theirinoperative positions are shown dotted in FIG. 3. They are movable bymeans of solenoids to the work station position 41 when required. Alsoshown dotted in FIG. 3 is the position of a Scorotron 44 which is a gridcontrolled Scorotron provided for charging the disc. The toner 42 andfuser 43 are attached to the static framework of the recorder whilst theScorotron is attached to the transverse wall 24 so that it travelstherewith.

FIG. 4 shows a side elevation of part of an optical system for use inthe apparatus of FIG. 3. The optical system comprises an upper unit 45positioned above the disc 1 when the apparatus is in use, opticallyaligned with a lower unit 46 positioned below the disc.

The upper unit 45 illuminates the disc during information retrieval orreading. It comprises a casing 47 holding a lamp 48, in an ellipsidalreflector, and a mirror 49 for reflecting light from the reflectortoward the disc. The casing further contains upper and lower adjustableoptical filters 50 and 51 placed respectively above and below afour-element lens assembly 52 for providing suitable illumination. Theupper filter may be a spatial filter for producing either a lightpattern (via a small hole in the filter) suitable for reading of thedigital control data track on the disc or a light pattern (via a largerhole in the filter) suitable for reading analogue data contained in thedisc. The lower filter may be a filter offering either an infra-redfilter (via which the disc is illuminated during reading) or a hotmirror (blanking off the light from above during recording). The lowerunit 46 focusses optical information during recording and retrieval. Itcomprises a casing 53 containing a Gauss lens system 54 and pivotablemirror 55. The Gauss lens system 54 is movable perpendicular to the discto permit focussing. When the apparatus is used for recording, opticalinformation is projected (by means not shown in FIG. 4) onto the mirror55 and reflected upwardly through the Gauss lens system which focussesit into the disc. When the apparatus is used for retrieval a page ofinformation on the disc is illuminated by the upper unit 45, and theGauss lens system 54 and the mirror 55 serve to transmit the informationto information display or data reading means (not shown in FIG. 4). Theoptical system of FIG. 4 also includes air bearings 151 and 153 betweenthe upper optical unit 45 and the disc 1 and the lower optical unit 46and the disc. These bearings serve to prevent contact between theoptical system and the disc. This also serves to purpose, in recordingin an electrophotographic system, of preventing electrical discharge ofthe disc. The bearing shown may be an aerostatic or hydrostatic bearing,in which a fluid is pumped through slots or holes in rings surroundingthe periphery of the optical system at faces adjacent to the disc. Thefluid pressure maintains a separation between the disc and the opticalsystem.

In the bearing of FIG. 4, the upper bearing comprises an air-bearingring 151 slidably mounted on pins on a second ring 152, so as to be freeto move in a direction perpendicular to the plane of the disc 1. Thusthe upper bearing comprises a floating bearing. The lower bearingcomprises an air-bearing ring fixedly mounted on the lower optical unit46.

Such an arrangement allows adjustment of separation between the disc andthe lower optical unit 46 by a simple adjustment of pressure in both thebearing ring 151 and the bearing ring 153. Such an adjustment ofseparation may be used, for example, for fine adjustment of focussing ofthe optical system.

FIG. 5 shows optical paths in an optical system for use in the device inFIG. 3, including recording and image display means.

When the system is used for recording analogue pages, the information tobe recorded is placed on an illuminated plate 56. Light travelling fromthe plate is reflected by a mirror 57 onto a movable mirror 58, movedinto a first recording position for receiving light from the mirror 57.The mirror 58 reflects the light onto the mirror 55 of the lower unit46, which transmits the light through the Gauss lens system 54 (notshown in FIG. 5), onto the disc 1 (only a part of which is shown) inFIG. 5.

When the system is used for analogue information retrieval, the lamp 47of the upper unit 46 transmits light to the mirror 49, which reflectsthe light through the four lens assembly 52 and the filters 50 and 51(not shown in FIG. 5), to impinge on the disc.

An image of the illuminated portion of the disc is focussed by the Gausslens system 54 (not shown), and reflected by the mirror 55 onto themovable mirror 58, moved into a second, retrieving position. The movablemirror 58 reflects the image onto a mirror 59, which in turn reflects inonto a mirror 60. This mirror 60 reflects the image onto a displayscreen 61, only part of which is shown for the sake of clarity.

it is desirable to ensure in the optical system that there is anodd-number difference between the number of mirrors in the recordingsystem and the number of mirrors in the retrieval system below the disc.This is to avoid image inversion.

For digital reading and recording the mirror 55 is rotated 90° about itsvertical axis to a position shown in dotted lines in FIG. 5.

During reading, light reflected from the mirror 55 passes through a lens91 and a beam splitter 90 and impinges upon a photocell array 62 usedfor reading of digital information.

For recording digital information, the required bit pattern is producedby a LCD array 92. This array is illuminated by a lens and lamparrangement 93. The illuminated image of the array is reflected by amirror 94 into the beam splitter 90. This transmits the light to themirror 55 by way of the lens 91. The digital image is then focussed onthe disc for recording in a manner similar to that used in analogueimage recording.

FIG. 6 is a schematic circuit diagram of an electronic control systemfor the apparatus of FIG. 3.

This is based upon an MC6809 microprocessor chip and detailedinformation on usage can be found, for example, from "An introduction toMircrocomputers", Volume II, Some Real Products by Adam Osborne andAssociates. Motorola data sheets also provide detailed instructions.Accordingly FIG. 6 shows only a schematic circuit diagram.

The microprocessor circuit itself is designated 107 in FIG. 6 and iscoupled to a system data bus 108. Also coupled to the data bus, viaaddressing and interface logic 110, is a read/write memory 109. A readonly memory 112 is also coupled to the data bus by way of addressing andinterface logic 113. A 6840 timer 114 is also coupled to the data busfor the purpose of timing during serve control. Input/output ports 115are also provided, being coupled to the data bus by interface logic 116.Coupled to the ports 115 are moire fringe transducers 35 and 39, akeyboard 105, the photocell array 52, and the liquid crystal array 92.The moire fringe transducers are coupled to the ports by an up-downcounter 120. Also coupled to the input/output ports are limit switchesfor detecting the limits of radial travel.

The data bus also communicates with an eight-bit, plus sign bit,digital-to-analogue converter 117 controlling a power amplifier 118which drives motors 33 and 27 via a multiplexer/changeover switch 119.

Motors 33 and 27 are shown coupled to the moire fringe transducers viadotted lines in FIG. 6, this representing the optical coupling via theco-ordinate scales.

Also coupled to the bus are a fixer drive 151, a toner drive 152, and aScoratron control 152.

The following description relates to the development of images in theapparatus, using TEP material, requiring toning and fixing.

In order to achieve correct toning, three conditions are ideally met:

(1) the time for which each part of the image is exposed to the tonershould be substantially the same;

(2) the flow velocity of toner over the latent image should not besufficient to produce spurious tribo-electric charge; and

(3) the toner application must present a flat equipotential surface inwhich irregularities are sufficiently small not to cause fielddistortions in the region of the image.

One form of toner applicator which aims to meet these conditions isillustrated in FIGS. 7 and 8. In these drawings, the toner applicator isindicated generally at 202. Toner is applied through a channel 203 andfills the space between applicators 202 and disc 1. Suction is appliedthrough a channel 201 which limits the quantity of toner in theoperating space so that it covers an area just sufficient in size (thelimit is shown in FIG. 8 at meniscus 205). After a sufficientdevelopment time, suction is applied to channel 203, until most of thetoner has been withdrawn. Air is then blown through channel 201,sweeping the remainder of the toner towards channel 203. A small drop oftoner may remain opposite channel 203, and this small amount of tonermay be ignored or steps may be taken to assist its removal. If the dropof toner is ignored, the end result is that, when channel 201 and 203cease to be active, the droplet will spread into a uniform thin layerwhich may be thin enough not to degrade the final image. Instead oftaking this risk, the droplet of toner may be diluted by the addition ofseveral drops of liquid, for example the liquid component of the toner(e.g. a paraffin) or a volatile liquid which is miscible with the toner.The dilute liquid is then sucked away by applying suction to channel203, which again is effective except for a small last droplet. Beingmore dilute than the original droplet, the effect of the remainingdiluted droplet is insignificant. Alternatively, a pointed probe may beinserted through channel 203 and made to contact the drop, which willthereby cling to the probe in preference to the surface of disc 1. Ifdesired, this technique may be used after the dilution technique alreadydescribed.

It is preferred to minimise any turbulence which may occur during theapplication of toner liquid. An improvement in this respect can beobtained by making the outlet of channel 203 concavely curved, i.e.bell-mouthed.

In practice, the time required for development of the image may be ofthe order of 1 second. In the course of this time, the flow speed of thetoner must be restricted to avoid tribo-electric effects. Also, the flowfrom a single orifice such as the outlet of channel 203 may take asignificant fraction of one second in order to fill the operatingvolume. These limitations may be avoided by modifying the orifice ofchannel 203 so as to give a distributed arrangement. A wide choice ofgeometrical arrangements can be used to give a distributed orifice, andthe choice depends on the degree of distribution required, the materialsinvolved and the manufacturing technique employed to form the orifice.Suitable configurations are circular holes, and linear or annular slits;these can all be achieved by simple production processes. FIGS. 9 and 10show two arrangements in which the orifice of channel 3 is extended intoa slit-like form. The circumference of the slit is much greater thanthat of a small circular hole and this results in a reduction in thevelocity of toner in the course of its distribution. The slit can alsobe made narrow compared with the separation between toner applicator anddisc, whicn minimises field distortion.

The arrangement of FIG. 10 shows an interdigitated arrangement whichallows very rapid application and extraction of toner. Because of thelarge linear extent of the outlet slits, their width can be extremelynarrow. An interdigitated arrangement of this sort can be used wherevery high image quality is required, with most faithful rendering ofintermediate tones.

The toner applicator of FIG. 11 is particularly suitable for applyingtoner for use in recording of optical images. The applicator isindicated generally at 80. Toner is applied through a line of transversecircular-section channels 81 and fills the space between the planarupper surface 82 of the applicator and the surface of the disc 1.

These channels have a similar effect to a single transverse slot.

After a sufficient development time (about 1 second) excess toner issucked out through the same channels 81 through which it was applied. Ithas been found that this method of removal of toner may be particularlyeffective.

The applicator of FIG. 12, indicated generally at 83, is particularlysuitable for applying toner for use in recording digital information.The applicator has a projecting centre section 84 having a smallelongate, planar upper face 85. The long axis of the upper face is in adirection tangential to the disc when the applicator is in use.

Toner is applied through a longitudinal line of channels 86 and fillsthe space between the upper face 85 and the disc. After a sufficientdevelopment time, excess toner is sucked out through the channels 86 bywhich it was applied. The shape and small size of this applicator makesit particularly suitable for use with digital information recording.

The toner applicator moving equipment of FIG. 13 comprises the base 14having two uprights 65, 66 supporting linear guide rails 67, 68 whichare in spaced-apart parallel relationship. On these guide rails issupported a carriage 69. The carriage includes an upright wall 70 towhich is attached one end of a solenoid 71. The other end of thesolenoid 71 is attached to the upright 66.

Rigid members 72, 73 are pivotally mounted on the carriage. Remote endsof the members are pivotally mounted on the toner applicator 42, so asto provide a parallelogram linkage between the applicator 42 and thecarriage. The lower rigid member 73 further includes a free portion 74extending from the carriage in a direction approximately opposite tothat of the work-station, which free portion includes a forked member 75attached to a lower part of the free portion.

Fixed on the base 14 is a second solenoid 76 the shaft 77 of which is ofa diameter such as to be able to pass between the forks of the forkedmember. A collar is mounted on the upper end of the shaft, which collarhas a diameter sufficient to prevent it from passing between the forksof the forked member.

The equipment moves the applicator between an inoperative and anoperative position.

The the applicator is in an inoperative position, the equipment is asshown in FIG. 13. The solenoid 71 is in its contracted state, with thecarriage 69 to a rightward position (as seen in FIG. 13) and hence withthe applicator 42 out of line with the axis of rotation of the disc,indicated by the broken line 78. In this position the forked member isout of engagement with the shaft 77 of the solenoid 76, and hence theapplicator is lowered out of proximity with the disc 1.

To move the applicator into its operative position, the solenoid 71 isenergised, causing it to move to its expended state, pushing thecarriage 69 along the guide rails 67, 68 and aligning the applicatorwith the axis of the disc. This motion also causes the forked member 75to engage with the shaft of the solenoid 76. The solenoid 76 is thenenergised, causing its shaft to move downward. This causes the collar ofthe shaft to abut against the forked member and move that member alsodownward. Thus, the applicator 42 is raised into close working proximitywith the disc 1.

The operation of the system will now be described.

The memory 112 contains programme data which defines the fundamentaloperation of the recording apparatus, including defining functions forkeys of the keyboard 105. In FIG. 6 two of these keys are designated Land S.

The operator will initially, having applied power to the system andloaded a disc, actuate key S.

On hitting the key S, a starting sequence will be initiated as follows.

Firstly, the disc will be displaced so that the optical path 40 will beat its radially outermost position, detected by a limit switch 97' (FIG.6). With mirror 55 in its digital data position, motor 27 is operated tomove the optical path 40 radially inwardly to detect track 5,specifically so as to locate the extra wide guard rail 9. At this stage,data from the photocell array 62, with decoder, will be received by themicroprocessor 107. The width of this array is more than sufficient toencompass the radial width of band 5, so that, within certain limits ofeccentricity, the whole width of the band can be imaged onto thephotocell array. According to the amount of eccentricity at anyparticular angle, a different set of photocells will have the image ofbit sets 17 imaged upon them. The microprocessor will be able to detectwhich cells, if any, have the guard rails 9, 10 imaged upon them andfrom this will determine three factors. Firstly, it will determinewhether or not both rails are within the range of the array, and, ifnot, will adjust the radial position of the disc accordingly via motor27. Secondly, it will determine the actual value of eccentricity at thatgiven angular position and will store that value in memory 109. Thirdly,it will be able to ascertain which photocells are in a positioncorresponding to the positions of the twenty bits of a set 17. Theseparticular photocells will be read by the microcomputer to establish theindividual bit values. In particular, if it is ascertained that, say, aknown group of eight photocells of the array have a positioncorresponding to a given bit, then, say, the output from the middle fourphotocells of that set will be averaged to obtain the bit value. Thisprovided an absolute angular position in the angular co-ordinate system.Further angular position measurement can now be effected by way of themoire fringe transducer 35, because this transducer operates on track 6when the optical path is withdrawn away from the track. Similarly, theposition of track 5 (or one of its guard rails 9 and 10) will provide anabsolute address for the radical co-ordinate by which the `zero` of thescale 38 can be determined.

The next step in the process will be for the optical path 40 to be movedunder servo control to reach innermost track 4 (or a given one oftracks--defined by detecting the second extra wide band 12) and read thesets of data bits contained therein in a serial manner.

In reaching the data track 4, the reading head will also scan innerguard rails 12. With the aid of the moire fringe transducer 39 and scale38, the inner and outer guard rails 9 and 12 permit the microprocessorto calculate a fixed radius measurement of the disc and so calibrate theradial measuring system of elements 38 and 39. Such a radius measurementcan be established at several known angular positions to measure anydisc distortion radial and to store that distortion for addresscorrection, in addition to any eccentricity correction. A first group ofcharacters or words in selected track 4 will be read into themicroprocessor system so that the system may ascertain the type of disc,in particular, whether or not any index track is provided, whether ornot any programming track is provided and whether or not there is any"title" or "menu" page to be accessed initially. It is pointed out herethat one or more tracks 4 may be provided with programme data whichdefines the manner in which the reader should response to data indexinginformation, and such programming information will be read into memory109 for subsequent use during reading of the disc. The memory will alsohave read into the contents the track or tracks 4 containing indexinginformation regarding pages already recorded on the disc.

If the data read from the track or tracks 4 defines an initial page tobe displayed, then the optical path will be moved radially to therequired radial co-ordinate for that page, using the moire fringetransducer 39 to measure radial distance moved. For the present it isnoted that when this stage has been reached, the optical system willhave been set to image the page to be displayed, the microprocessorsystem having ceased reading data from the photocell array 62, and willhave rotated mirror 55 to enable that page to be displayed by way of theoptical system (48, 49, 55 and 58 to 61) shown diagrammatically in FIG.5. The disc will then be rotated under servo control the angulardistance necessary to reach a position corresponding to the knownangular co-ordinate of that page. Such radial and angular servo controlpositioning is under control of the moire reading heads 35 and 39 inconjunction with the motors 27 and 33 and this control will be describedin more detail hereinafter. The operator may select further pages byactuating keys on the keyboard.

In one scheme, the individual pages have page numbers running from, say,1 to 9000 and the operator may input that page number to access thecorresponding page. The microprocessor will decode that page number intoa corresponding radial co-ordinate value and angular co-ordinate valuefor page searching in the manner briefly described above.

An alternative method of access is via indexing, and this process willbe described hereinafter.

The way in which the servo positioning system operates upon motors 32and 33 will now be described in more detail.

Firstly it will be apparent that the system must incorporate two servosone for controlling the rotational position of the disc by motor 33 andthe other for controlling the radial position of the reading head bymotor 27. The only difference between the two servos is the value of thecoefficients used by an algorithm within the microprocessor system forcalculating motor control voltage. These coefficients will be stored inthe memory 112. Motor control data is supplied to the digital-to-analogconverter 117 which controls the power amplifier 118 so that it shoulddeliver the desired control voltage with the correct polarity or sign.Radial or angular control is selected by the multiplexer or switch 119.

Because each servo is fundamentally the same, that applicable tocontrolling angular motion only will be described in detail.

From the information available to the microprocessor, it knows at anyone moment the current angular position of the disc and the desiredangular position and accordingly knows the distance of angular travelrequired. If that distance is extremely small, only fine-control isapplied, but when that distance is greater than a given value, then thecontrol of position is effected firstly by coarse-control followed byfine-control.

In the coarse-control operation, the timer 114 plays a part ingenerating an interrupt every 8.9 ms, this time interval being the mainsampling period for the servo system. The coarse-control algorithm ofthe system is in two parts, firstly an acceleration phase in which fullmotor voltage is applied by the power amplifier, and a decelerationphase in which a deceleration voltage is applied by the power amplifierof such a value as to stop the motor with the disc having reached a"window" around the final target.

Further details of one example of servo control may be found in EuropeanApplication No. 81300321.7.

It has been indicated above that a digital track or tracks 4 could beprovided so as to contain indexing information. Moreover, digitalprogramming information can be stored to provide the apparatus withsoftware particularly adapted to handle the indexing data that isprovided on the same disc. In this way a variety of types of indexes canbe made compatible with a single embodiment of disc recorder by havingthe apparatus effectively reprogrammed in part each time a disc isloaded.

For example, a document to be recorded may be assigned only one, or morethan one index term. As a further option a menu of index terms may beprovided, arranged alphabetically or in a structured manner. In yet afurther option the index terms may be predetermined or arranged to beupdated at will. As a fifth option, access may be by a word(alphanumeric string) or by a numeric code. A sixth option involvessotring digital index data as a sequential list (in document order, thatis document identified follow by index terms) or as an inverted list(index term followed by document identified).

Three possible indexing systems will now be described. The basicdifferences between the systems are firstly the type of access; whetherby word or number code, and secondly the method of storing the data;whether sequentially, in order of documents, or as an inverted list,listed by index term.

A type one system comprises an inverted indexing system with numericalcode access. Searching is performed using an optical menu. This systemallows a document to be assigned to more than one index. It furtherpermits searching under more than one key. New keys can be added asnecessary up to a maximum dictated by the number of datablocksavailable.

Adding keys, however, requires production of a new optical menu page.

A type two system offers access by word, with an inverted indexingsystem. It is similar to the type one system, except that it maintainsthe menu in digital form. When the user types in a keyword, the systemsearches for it on the digital "menu page", retrieves the address of thedatablock associated with it, and then retrieves the addressed ofdocuments listed in that datablock.

A type three system has access by word, but has sequential indexing. Theindex is digitally recorded. In such a system a document may be assignedto more than one key, and may be searched using more than one key, withBoolean operators. The sequential index permits addition of keys. Withsuch a system, at least some of the digital data must be read intomicroprocessor memory for filing, and all of it for retrieval. Inoperation, the user keys in a word, the system searches a list for theword, finds the 16-bit code associated with that word, and then searchesfor that code in the sequential index.

During recording the keyboard is use by the operator to enter thenecessary index data, which is recorded as digital data, as describedabove, and as determined by a programme contained in the recordingapparatus memory and at least part of which may have been retrieved fromthe disc, as also described above.

A VDU, or simple display, may also be coupled to the processor to guideand inform the user.

We claim:
 1. A recording apparatus for recording optically readableimages on a storage member comprising recording material by means ofwhich such images can be recorded on the member, the apparatuscomprising processing means for recording said images on the storagemember, the processing means having imaging means for imaging data to berecorded onto the storage member, and the apparatus also comprisingdrive means for producing relative motion between the imaging means andthe storage member so that an image can be directed at a desiredlocation of the storage member, and control means for controlling thedrive means to enable an image to be recorded at any desired one of aplurality of areas of a two-dimensional array of areas in a first zoneof the storage member, the control means comprises means for readingco-ordinate defining positional information conveyed by the storagemember in a second zone to ascertain a datum position for a firstco-ordinate and for a second co-ordinate of a two-dimensionalco-ordinate system in which each of said areas has its owntwo-dimensional co-ordinate address; and said control means comprisingmeasuring means for measuring relative displacements in the twoco-ordinate directions to establish co-ordinate addresses relative tothe datum positions of the co-ordinates.
 2. An apparatus as claimed inclaim 1, for use with recording material of a kind with which theimaging means forms a latent image, the apparatus comprising means forrendering said image substantially permanent.
 3. An apparatus as claimedin claim 2, for electrophotographic recording material, the renderingmeans comprising means for electrically charging said material, andmeans for image toning, and image fixing.
 4. An apparatus as claimed inclaim 3, and comprising a mechanism, coupled to be controlled by thecontrol means, for moving the toning means and fixing means sequentiallyto a work station position.
 5. An apparatus as claimed in claim 1,wherein the imaging means comprises an optical input path by which areduced image of a document of legible size can be formed for imaging atthe storage member.
 6. An apparatus as claimed in claim 1, wherein themeasuring means comprises means for measuring displacements in oneco-ordinate direction by sensing a positional data on the storagemember.
 7. An apparatus as claimed in claim 6, wherein the measuringmeans comprises means for measuring displacements in the otherco-ordinate direction by sensing positional data which is part of theapparatus and distinct from the storage member.
 8. Apparatus as claimedin claim 1 wherein the processing means includes means for recordingdigital data on the storage member in addition to the data recorded inthe array of areas.
 9. Apparatus as claimed in claim 1 and includingreading means for reading digital data recorded on the storage member inaddition to data in the array of areas.
 10. Apparatus as claimed inclaim 9, wherein the reading means (62) includes means for producingelectrical signals for use by the control means on the basis of digitaldata read.
 11. An apparatus as claimed in claim 1 characterised in thatthe apparatus comprises means (28,29) for rotatably mounting the storagemember.
 12. Apparatus as claimed in claim 1, characterised in that thetwo-dimensional co-ordinate system is a polar-co-ordinate system.
 13. Arecording apparatus for recording optically readable images on a storagemember comprising recording material by means of which such images canbe recorded on the member, the apparatus comprising processing means forrecording said images on the storage member, the processing means havingimaging means for imaging data to be recorded onto the storage member,and the apparatus also comprising drive means for producing relativemotion between the imaging means and the storage member so that an imagecan be directed at a desired location of the storage member, and controlmeans for controlling the drive means to enable an image to be recordedat any desired one of a plurality of areas of a two-dimensional array ofareas in a first zone of the storage member, the control means comprisesmeans for reading co-ordinate defining positional information conveyedby the storage member in a second zone distinct from the first zone toascertain a datum position for a first co-ordinate and for a secondco-ordinate of a two-dimensional co-ordinate system in which each ofsaid areas has its own two-dimensional co-ordinate address; and afluid-pressure operated bearing for preventing contact between thestorage member and the imaging means when the apparatus is in use. 14.Apparatus as claimed in claim 13, wherein there are means forselectively adjusting the fluid pressure provided by the bearing toadjust the spatial separation between the imaging means and the storagemember and so provide image focussing.
 15. A recording apparatus forrecording optically readable images on a storage member comprisingrecording material by means of which such images can be recorded on themember, the apparatus comprising processing means for recording saidimages on the storage member, the processing means having imaging meansfor imaging data to be recorded onto the storage member, and theapparatus also comprising drive means for producing relative motionbetween the imaging means and the storage member so that an image can bedirected at a desired location of the storage member, characterized inthat there are control means for controlling the drive means to enablean image to be recorded at any desired one of a plurality of areas of atwo-dimensional array of areas in a first zone of the storage member,the control means comprises means for reading co-ordinate definingpositional information conveyed by the storage member in a second zonedistinct from the first zone to ascertain a datum position for a firstco-ordinate and a second co-ordinate of a two-dimensional co-ordinatesystem in which each of said areas has its own two-dimensionalco-ordinate address; and said control means including compensation meansfor compensating for any error between at least one of the twoco-ordinates of the storage member and the corresponding co-ordinate asdefined by the drive means.
 16. Apparatus as claimed in claim 15,wherein the compensation means include means for reading compensationreference data recorded on the storage member.
 17. An apparatus asclaimed in claim 16, wherein the compensation means comprises means forreading a track extending in one co-ordinate direction to detect anyskew that may exist between the measuring means and that co-ordinatedirection on the storage member.
 18. An apparatus as claimed in claim16, wherein the compensation means comprises means for reading twoelements on the storage member spaced apart in the direction of one ofthe coordinates to provide a reference length measurement in thedirection of that on co-ordinate for use in isotropic scale errorcompensation.
 19. An apparatus as claimed in claim 18, wherein thereading means are arranged to any variations in the spacing of thosetracks for use in anisotropic scale error correction.
 20. A recordingapparatus for recording optically readable images on a storage membercomprising recording material by means of which such images can berecorded on the member, the apparatus comprising processing means forrecording said images on the storage member, the processing means havingimaging means for imaging data to be recorded onto the storage member,and the apparatus also comprising drive means for producing relativemotion between the imaging means and the storage member so that an imagecan be directed at a desired location of the storage member,characterized in that there are control means for controlling the drivemeans to enable an image to be recorded at any desired one of aplurality of areas of a two-dimensional array of areas in a first zoneof the storage member, the control means comprises means for readingpositional information conveyed by the storage member in a second zonedistinct from the first zone to ascertain a datum position for a firstco-ordinate and a second co-ordinate of a two-dimensional co-ordinatesystem in which each of said areas has its own two-dimensionalco-ordinate address; and said apparatus including means for readingindexing information conveyed by the storage member in a zone distinctfrom the first zone to obtain the two-dimensional address of desireddata already recorded in the first zone.
 21. A recording apparatus forrecording optically readable images on a storage member comprisingrecording material by means of which such images can be recorded on themember, the apparatus comprising processing means for recording saidimages on the storage member, the processing means having imaging meansfor imaging data to be recorded onto the storage member, and theapparatus also comprising drive means for producing relative motionbetween the imaging means and the storage member so that an image can bedirected at a desired location of the storage member, characterized inthat there are control means for controlling the drive means to enablean image to be recorded at any desired one of a plurality of areas of atwo-dimensional array of areas in a first zone of the storage member,the control means comprises means for reading positional informationconveyed by the storage member in a second zone to ascertain a datumposition for a first co-ordinate and a second co-ordinate of atwo-dimensional co-ordinate system in which each of said areas has itsown two-dimensional address; and input means for inputting indexing datato the control means, the apparatus being arranged to record in a zoneof the storage member distinct from the first zone the indexing dataplus the co-ordinate address of the corresponding recorded area in thefirst zone.
 22. A recording apparatus for recording optically readablemulti-bit images on a storage member comprising recording material bymeans of which such images can be recorded on the member, the apparatuscomprising processing means for recording said images on the storagemember, the processing means having imaging means for imaging amulti-bit image to be recorded onto the storage member, and theapparatus also comprising drive means for producing relative motionbetween the imaging means and the storage member so that an image can bedirected at a desired location of the storage member, characterized inthat there are control means for controlling the drive means to enablean image to be recorded at any desired one of a plurality of areas of atwo-dimensional array of areas in a first zone of the storage member,the control means comprises means for reading positional informationconveyed by the storage member in a second zone to ascertain a datumposition for a first co-ordinate and to a second co-ordinate of atwo-dimensional co-ordinate system in which each of said areas has itsown two-dimensional co-ordinate address; and said apparatus includingoptical reading means for optically displaying any desired one of theplurality of areas in the first zone of the storage member.
 23. Anapparatus as claimed in claim 22 and comprising input means by which theuser can enter data for use by the control means in conjunction withsaid indexing information to determine the address of an areacorresponding to the entered data.
 24. An apparatus as claimed in claim23, wherein the control means are operable to determine the address ofan area from a defintion, entered at the input means, in the form of alogical combination of names, or codes representing names, of classes ofinformation.
 25. A storage member (1) comprising recording material bymeans of which optically readable images (8) can be recorded and storedby the member, characterised in that the member comprises a first zone(7) at which said recording material is provided and, distinct from thefirst zone, means which provide a machine readable datum (5,6) for afirst co-ordinate and for a second co-ordinate of a two co-ordinatesystem providing addressing means for each of a plurality of areas ofthe first zone (7).
 26. A storage member as claimed in claim 23, andcomprising machine readable digital data recorded in a zone (4) distinctfrom the first zone (7).
 27. A storage member as claimed in claim 24,wherein the digital data comprises addresses of areas of the first zone(7) associated with respective data defining a class of information 28.A storage member as claimed in in claim 23 wherein the recordingmaterial is electro-photographic.
 29. A storage member as claimed inclaim 23 wherein the recording material is photographic.
 30. A storagemember as claimed in claim 23 wherein the recording material isphotochromic.
 31. A storage member as claimed in claim 23 wherein thedatum means comprises an absolute address zone defining a series ofabsolute addresses for one co-ordinate and extending in the direction ofthat co-ordinate and extending in the direction of that co-ordinate. 32.A method of storing information on a disc, which comprises (a)sensitizing a predetermined zone of the disc so that it becomessensitive to electromagnetic radiation in, or close to, the opticalrange of wavelengths; (b) focussing onto the said predetermined zone, ora part thereof, a reduced image of the information to be recorded; (c)developing and/or fixing the predetermined zone or said part thereof soas to hold the reduced image in reproducible form within or on thematerial constituting the predetermined area of said disc; and (d)providing positional address data on the disc in one or more areasseparate from said predetermined zone to enable the information recordedat said zone to be retrieved.
 33. A recording apparatus for recordingoptically readable multi-bit images on a storage member comprisingrecording material by means of which such multi-bit images can berecorded on the member, the apparatus comprising processing means forrecording said images on the storage member, the processing means havingimaging means for imaging a multi-bit image, in a single imaging step,onto the storage member, and the apparatus also comprising drive meansfor producing relative motion between the imaging means and the storagemember so that a multi-bit image can be directed at a desired locationof the storage member and control means for controlling the drive meansto enable a multi-bit image to be recorded at any desired one of aplurality of areas of a two-dimensional array of areas in a first zoneof the storage member, the control means comprising means for readingco-ordinate defining information conveyed by the storage member in asecond zone separate from the first zone to ascertain a datum positionfor a first co-ordinate and for a second co-ordinate of atwo-dimensional co-ordinate system in which each of said areas has itsown two-dimensional co-ordinate address, and means for use in measuringrelative displacements of the imaging means and storage member in thetwo co-ordinate directions to establish co-ordinate addresses relativeto the datum positions of the co-ordinates.
 34. An apparatus as claimedin claim 33 wherein the means for use in measuring relativedisplacements comprises means for reading positional data on the storagemember in at least one co-ordinate direction.
 35. An apparatus asclaimed in claim 34 wherein the reading means is arranged to read anincremental scale of the storage member in order to measure incrementsof relative displacement in one of the co-ordinate directions.
 36. Anapparatus as claimed in claim 35, wherein the reading means comprisesmeans for reading co-ordinate measurement data on the storage member forboth co-ordinate directions.
 37. An apparatus as claimed in claim 35comprising measuring means for measuring relative displacements in theother co-ordinate direction by sensing positional data which is part ofthe apparatus and distinct from the storage member.
 38. An apparatus asclaimed in claim 33 wherein the control means include compensation meansfor compensating for an error between at least one of the twoco-ordinates of the storage member and the corresponding co-ordinate asdefined by the drive means.
 39. An apparatus as claimed in claim 34wherein the compensation means include means for reading compensationreference data recorded on the storage member.
 40. An apparatus asclaimed in claim 39 wherein the comgensation means comprises means forreading a track extending in one co-ordinate direction to detect anyskew that may exist between the measuring means and that co-ordinatedirection on the storage member.
 41. An apparatus as claimed in claim 39wherein the compensation means comprises means for reading two elementson the storage member spaced apart in the direction of one of thecoordinates to provide a reference length measurement in the directionof that one coordinate for use in isotropic scale error compensation.42. An apparatus as claimed in claim 41, and comprising the meansarranged to respond to any variations in the spacing of said twoelements for use in anisotropic scale error correction.
 43. An apparatusas claimed in claim 33 wherein the processing means includes means forrecording digital data on the storage member in addition to the datarecorded in the array of areas.
 44. An apparatus as claimed in claim 33and comprising input means for inputting indexing data to the controlmeans, the apparatus being arranged to record the indexing data plus theaddress in the first zone of the corresponding recorded area.
 45. Anapparatus as claimed in claim 44 wherein the apparatus is arranged torecord said indexing data in a zone distinct from the first zone.
 46. Anapparatus as claimed in claim 45 and comprising indexing data as sets ofdigital bits, each set being imaged as a multi-bit image in one step.47. An apparatus as claimed in claim 33 wherein the two-dimensionalco-ordinate system is a polar co-ordinate system.
 48. An apparatus asclaimed in claim 33 for use with recording material of a kind with whichthe imaging means forms a latent image, the apparatus comprising meansfor rendering said image substantially permanent.
 49. An apparatus asclaimed in claim 48 for electrophotographic recording material, therendering means comprising means for electrically charging said materialand means for image toning and image fixing.
 50. An apparatus as claimedin claim 49 and comprising a mechanism coupled to be controlled by thecontrol means, for moving the toning means and fixing means sequentiallyto a work station position.
 51. An apparatus as claimed in claim 33wherein the imaging means comprises an optical input path by which areduced image of a document of legible size can be formed for imaging atthe storage member.
 52. An apparatus as claimed in claim 33 furtherincluding a fluid pressure operated bearing for preventing contactbetween the storage member and the imaging means when the apparatus isin use.
 53. An apparatus as claimed in claim 52 wherein there are meansfor selectively adjusting the fluid pressure provided by the bearing toadjust the spatial separation between the imaging means and the storagemember and so provide image focusing.
 54. An apparatus as claimed inclaim 53 and including reading means for reading digital data recordedon the storage member separately from the array of areas.
 55. Anapparatus as claimed in claim 54 wherein the reading means includesmeans for producing electrical signals for use by the control means onthe basis of digital data read.
 56. An apparatus as claimed in claim 33including means for reading indexing information conveyed by the storagemember in a zone distinct from the first zone to obtain thetwo-dimensional address of desired data recorded in the first zone. 57.An apparatus as claimed in claim 56 including optical reading means foroptically displaying any desired one of the plurality of areas in thefirst zone of the storage member.
 58. An apparatus as claimed in claim57 and comprising input means by which the user can enter data for useby the control means in conjunction with said indexing information todetermine the address of an area corresponding to the entered data. 59.An apparatus as claimed in claim 58, wherein the control means areoperable to determine the address of an area from a definition, enteredat the input means, in the form of a logical combination of names, orcodes representing names, of classes of information.
 60. An apparatus asclaimed in claim 33 wherein the apparatus comprises means for rotatablymounting the storage member.